Conventionally, an engine having a series supercharging system is known as an engine having a plurality of superchargers. In the series supercharging system, turbines of the superchargers rotated by receiving exhaust gas are arranged in series in one exhaust path, and compressors of the superchargers compressing intake air are arranged in series in one intake path.
A variable capacity type supercharger is also known in which exhaust gas is controlled suitably its flow rate and then introduced into a turbine constituting the supercharger so as to improve supercharging efficiency. A series supercharging system in which two superchargers compress intake air flowing in one exhaust path and at least one of the superchargers is variable capacity type is defined as a variable series supercharging system. The Patent Literature 1 discloses an engine having a variable series supercharging system.
Conventionally, in control of capacity in a variable capacity type supercharger, feedback control is performed with supercharging pressure detected by a supercharging pressure sensor. However, the supercharging pressure is indirect physical quantity about the action of the supercharger and is disadvantageous because the control of capacity of the supercharger cannot be performed accurately. Especially, the variable series supercharging system having the variable capacity type superchargers is disadvantageous because the control of capacity cannot be performed accurately further.    Patent Literature 1: the Japanese Patent Laid Open Gazette 2006-29110